In a general color picture tube, as shown in FIG. 5, an envelope includes a panel 2 having a face portion 1 whose front surface is substantially rectangular, and a funnel 3 joined to this panel 2. An inner surface of the face portion 1 is provided with a phosphor screen 4, and a shadow mask 5 is held so as to face this phosphor screen 4. Further, inside a neck portion 6 of the funnel 3, an electron gun 7 is provided. During the operation of such a color picture tube, three electron beams 8 arranged in an in-line manner are emitted from the electron gun 7, pass through apertures of the shadow mask 5 while being deflected by a magnetic field generated by a deflection device 9, which is attached to an outside of the funnel 3, and then are irradiated on the phosphor screen 4 so as to produce an image on the face portion 1.
In order to achieve a self-convergence configuration for converging the three electron beams to one point on the screen, the deflection magnetic field generated by the deflection device generally is distorted into a pincushion shape at the time of deflection in an in-line direction (in the following, referred to as a horizontal direction because this direction generally corresponds to a horizontal axis of the screen) and a barrel shape at the time of deflection in a direction perpendicular to the in-line direction (in the following, referred to as a vertical direction because this direction generally corresponds to a vertical axis of the screen). Therefore, the deflection magnetic field exerts a lens effect including a diverging effect in the horizontal direction and a converging effect in the vertical direction on the three electron beams passing through this deflection magnetic field. Since the deflection magnetic field intensifies in keeping with the amount of deflection, the above-mentioned lens effect increases toward a peripheral portion of the screen. Thus, even when a beam spot formed in a central portion of the screen is made into a perfect circle, beam spots formed in the peripheral (particularly, corner) portion of the screen are distorted to have a horizontally elongated shape. Moreover, over-focusing occurs in the vertical direction, so that a vertically-elongated low-brightness haze portion tends to be formed.
JP 61(1986)-99249 A discloses a technology for alleviating such over-focusing (referred to as a “first conventional technology”). FIGS. 6A and 6B show cross-sections, taken along a deflection direction, of a model in which an electron lens system generated by the difference in electric potential between electrodes in an electron gun in the first conventional technology is illustrated as in an optical lens and of paths of electron beams passing through this electron lens system, with the upper half showing a horizontal direction (H) and the lower half showing a vertical direction (V). FIGS. 6A and 6B show the electron lens system and paths 10 of the electron beams passing therethrough respectively in the central portion of the screen and the peripheral (corner) portion of the screen. Further, the left end of the figure indicates a crossover point of the electron beams corresponding to an object point of a lens system, while the right end thereof indicates a spot point on the screen corresponding to an image point of the lens system. An outgoing angle from the crossover point is expressed by θo, while an incident angle to the screen is expressed by θi.
As shown in FIG. 6A, in the central portion of the screen, the electron beams are focused by a main lens 11 alone. On the other hand, as shown in FIG. 6B, in the peripheral portion of the screen, a dynamic focus voltage according to an increase in the deflection angle is applied, thereby forming a four-pole lens 12 having a converging effect in the horizontal direction and a diverging effect in the vertical direction at a foregoing stage of the main lens 11 and weakening the main lens 11. The effect of the four-pole lens 12 cancels out the effect of a deflection magnetic field lens 13 by the deflection magnetic field, which intensifies toward the peripheral portion of the screen, and weakening the main lens 11 compensates for the difference in distance between the central portion and the peripheral portion of the screen, so that the electron beams come into just focus over the entire screen.
However, in the first conventional technology, although the electron beams can be maintained to achieve just focus both in the horizontal direction and the vertical direction, the electron beams have a large difference between an incident angle θih to the screen in the horizontal direction and an incident angle θiv to the screen in the vertical direction. In general, a magnification M of a lens system has a relationship of M ∝ (tanθo)/(tanθi) where θo is the outgoing angle from the object point to the lens system and θi is the incident angle from the lens system to the image point. Accordingly, (incident angle θiv to the screen in the vertical direction)>(incident angle θih to the screen in the horizontal direction) as in the first conventional technology illustrated in FIG. 6B results in (lens magnification Mv in the vertical direction)<(lens magnification Mh in the horizontal direction). In other words, since the lens magnification in the horizontal direction is larger than that in the vertical direction, the spot is distorted into a horizontally-elongated shape, causing a problem in that a horizontal dimension of the spot becomes so large as to lower a horizontal resolution and a vertical dimension of the spot becomes so small as to generate a moiré phenomenon.
A technology for solving such a problem is disclosed in JP 3(1991)-93135 A (referred to as a “second conventional technology”).
FIGS. 7A and 7B show a lens system and paths of electron beams according to the second conventional technology, as in FIGS. 6A and 6B. The central portion of the screen (see FIG. 7A) is similar to the first conventional technology (FIG. 6A), while in the peripheral (corner) portion of the screen (see FIG. 7B), a second four-pole lens 14 having a diverging effect in the horizontal direction and a converging effect in the vertical direction is formed further at the foregoing stage of the four-pole lens 12 formed in the first conventional technology. This second four-pole lens 14 allows the electron beams to diverge outward in the horizontal direction and converge inward in the vertical direction before reaching the main lens 11. As a result, the difference between the incident angle θiv to the screen in the vertical direction and the incident angle θih to the screen in the horizontal direction is reduced (in other words, the lens magnification in the horizontal direction and that in the vertical direction are made substantially equal in the peripheral portion of the screen). This makes it possible to bring the spot shape in the peripheral portion of the screen closer to a perfect circle, thereby both enhancing a horizontal resolution and suppressing the generation of moiré.
However, even in this second conventions technology, when the deflection angle increases excessively, there has been a problem that it becomes difficult to bring the spot shape in the peripheral portion of the screen closer to a perfect circle.
First, there is a problem that the horizontally elongated spot distortion in the peripheral portion of the phosphor screen cannot be corrected sufficiently due to an influence of a spherical aberration of the main lens. The reason follows. In the second conventional technology, when attempting to alleviate the horizontally-elongated spot distortion in the peripheral portion of the screen, the electron beams passing through the lens system travel close to an edge of the main lens 11, especially in the horizontal direction as shown in FIG. 7B. This phenomenon becomes noticeable as the deflection angle increases, i.e., the magnetic field intensifies. In this case, even when the electron beams ideally achieve the just focus as indicated by solid lines, they actually are affected by the spherical aberration that is noticeable at the edge of the main lens 11, so that the electron beams reaching the screen follow a path as indicated by a broken line and then are over-focused. As a result, the beam spots formed in the peripheral portion of the screen further are distorted into a horizontally-elongated shape, so that the spot dimension thereof tends to become too large.
In order to avoid the above, if attempting to bring the electron beam passing position in the main lens 11 in the horizontal direction as far inwardly as possible, the second four-pole lens 14 that serves to diverge the electron beams outward in the horizontal direction and converge them inward in the vertical direction becomes useless.
In other words, the conventional technologies have had a problem that, when the deflection angle increases excessively and the deflection magnetic field intensifies too much, the horizontally elongated spot distortion in the peripheral portion cannot be corrected sufficiently.